The development of higher recording densities in magnetic recording and playback devices is achieved by miniaturizing the recording track width and the recording bit length on the recording medium, but a result of this miniaturization is a decrease in the playback signal magnetic flux produced by recording bits on the recording medium. Therefore, in order to achieve a higher recording density, higher sensitivity for the sensor used in the playback head becomes necessary, and a highly sensitive tunneling magnetoresistive (TMR) film may be used.
Basically, a TMR film element is a structure formed by depositing a ground layer, an antiferromagnetic layer, a first ferromagnetic layer exchange-coupled to the antiferromagnetic layer, a second ferromagnetic layer which has a magnetic moment coupled anti-parallel to the magnetic moment of the first ferromagnetic layer through an anti-parallel coupling layer, an, insulation barrier layer, and a third ferromagnetic layer. The ferromagnetic layers of the first ferromagnetic layer, the anti-parallel coupling layer, and the second ferromagnetic layer are referred to as the pinned layer. The magnetic moments of the first ferromagnetic layer and the second ferromagnetic layer in the pinned layer have strong mutual anti-parallel coupling. Because the magnetic moment of the first ferromagnetic layer is strongly pinned by exchange coupling with the antiferromagnetic layer, the orientation of the magnetic moment of the pinned layer is not easily changed. On the other hand, the third ferromagnetic layer is referred to as the free layer. The direction of the magnetic moment of this layer is easily changed by a magnetic field applied externally.
The process of playing back the magnetic information recorded on the medium is as follows. When the signal magnetic field generated by a recording bit enters a sensor, the signal magnetic field causes the magnetic moment of the free layer to rotate. Therefore, the relative angle between the magnetic moments of the free layer and the pinned layer changes. When the relative angle changes, the resistance of the sensor changes in order to change the scattering probability due to the electron spin. The information of the recording bit is played back by transforming the change in resistance of the sensor into an electrical signal. For reference, FIG. 4 shows the playback process.
FIG. 5 presents a prediction example of the playback track width, the stripe height, and the area resistance RA which are required in order to respond to future increases in the surface recording density. In FIG. 5, the area resistance RA (hereinafter, referred to as RA) is the electrical resistance in the direction perpendicular to the film surface of a sensor having a 1 μm2 area. The resistance of a TMR sensor is inversely proportional to the area of the sensor because the current passes perpendicular to the film surface.
In order to miniaturize the playback track width and the stripe height accompanying the increase in the recording density, the area of the sensor passing the detection current becomes smaller. Consequently, when the RA is constant temporarily, the resistance increases inversely proportional to the area of the sensor. For example, in FIG. 5, when the recording density is increased from 350 Gb/in2 to 1000 Gb/in2, unfortunately, the area of the sensor becomes one-fourth, and the resistance is increased by four fold. When the resistance is increased by four fold in this way, the signal processing circuit system will no longer operate normally. Consequently, the resistance of the sensor seen from the signal processing circuit system must be constant. In other words, the area resistance RA of the sensor must be decreased as the sensor is miniaturized. In FIG. 5, the RA is calculated so that the resistance of the sensor becomes constant even as the area of the sensor decreases. As shown in FIG. 5, RA is 1.0 Ωμm2 at a surface recording density of 500 Gb/in2, which must decrease to an RA of 0.6 Ωμm2 at a surface recording density of 1000 Gb/in2, and must decrease further to an RA of 0.2 Ωμm2 at a surface recording density of 2000 Gb/in2. However, FIG. 5 shows only estimates. Although slight differences in the numerical values from the actual values are thought to arise, the future trends are represented.
In the description of Japanese Unexamined Patent Appl. Pub. No. 2006-80116, when MgO is created on a noncrystalline material Co—Fe—B alloy film by a sputtering method, MgO having good crystallinity may be produced, and a high MR ratio of 180% at room temperature is obtained by thermal processing. However, a problem with this technology is that the area resistance RA increases to 1000 Ωμm2, and the playback head resistance becomes too large for application to a playback head for a hard disk device.
With the objective of applying a TMR sensor using MgO to a playback head for a hard disk device, research has been conducted to improve the MR ratio in a low RA region. As a method for creating the insulation barrier layer, the proposal in K. Tsunekawa, D. D. Djayaprawira, M. Nagai, “CoFeB/MgO(001)/CoFeB Magnetic Tunnel Junctions for Read Head Applications,” Appl. Phys. Lett., 87, 072503 (2005) is a method which uses a magnesium oxide target to create a magnesium oxide layer via an RF sputtering method after a thin metallic magnesium layer of approximately 0.4 nm is deposited. By using this method, an MR ratio of 100% is realized at an RA of 2.0 Ωμm2.
As is clear from FIG. 5, however, the value needs to be decreased by a factor of 10 such that RA=0.2 Ωμm2 in order to produce the high recording density of 2000 Gb/in2. Even if the method disclosed in K. Tsunekawa, D. D. Djayaprawira, M. Nagai, “CoFeB/MgO(001)/CoFeB Magnetic Tunnel Junctions for Read Head Applications,” Appl. Phys. Lett., 87, 072503 (2005) is used, when the RA becomes less than 2.0 Ωμm2, the MR ratio decreases abruptly, and the MR ratio at RA=0.2Ωμ2 disappears. Therefore, when a known conventional TMR sensor using MgO is used in a 2000 Gb/in2 playback head, problems arise where the playback sensitivity is not obtained, and the head does not function as a playback head. The magnitude of RA becomes smaller as the film thickness of the insulation barrier layer MgO becomes thinner.
In FIG. 16 of W. H. Butler, X.-G. Zhang and T. C. Schulthess, “Spin Dependent Tunneling Conductance of Fe|MgO|Fe Sandwiches,” Phys. Rev. 8, 63, 054416-1 (2001), the film thickness of MgO and the theoretical calculation of the conductance are cited. The relationship to the MR ratio is the difference between the majority and anti-parallel conductances. When the number of MgO layers decreases, the difference between the majority and the anti-parallel conductances, which are related to the MR ratio, tends to decrease. When the majority and anti-parallel conductance lines are extended as straight lines, it is assumed that the MR ratio in three monolayers disappears for the most part because the lines intersect in the vicinity of the three monolayers. In investigations, the film thickness of the three monolayers of MgO corresponds to RA=0.3 Ωμm2. Consequently, the disappearance of the MR ratio at RA=0.3 Ωμm2 is believed to be the physical limit of the conventional TMR sensor when MgO is used in the insulation barrier layer.
In Japanese Unexamined Patent Appl. Pub. No. 2001-217483, the effects for a layered tunneling magnetoresistive effect element are the ability to form an insulation barrier layer having a completely oxidized metal layer without the oxidation affecting the ferromagnetic layers, and to form a thicker film for the insulation barrier layer. Also, the preferred effect is to use an oxide of at least one element selected from Al, Mg, Nb, Ni, Gd, Ge, Si, or Hf as the material of the insulation barrier layer.
In addition, the layered tunneling junction magnetoresistive effect element described in Japanese Unexamined Patent Appl. Pub. No. 2001-217483 uses a perovskite oxide, such as Ni, in the insulation barrier layer to correspond to using a perovskite oxide to form the ferromagnetic layers. The reasons are to relax the processing precision and to simplify the manufacturing process. This description does not suggest using an oxide other than a perovskite oxide, such as Ni, as the insulation barrier layer, and cannot expect the area resistance RA to decrease below 1.0 Ωμm2 even when this type of perovskite oxide is used as the insulation barrier layer.
Accordingly, a TMR head having a high MR rate of change even as the area resistance RA of the sensor decreases would be very beneficial in achieving higher density recording/playback magnetic heads for use in magnetic storage devices.